Why Dry Film Thickness Calculations for Liquid Waterproofing Mastics Are Frequently Incorrect
During practical installation of mastic waterproofing coatings, control measurements of the actual dry film thickness were carried out. The results showed a discrepancy between the measured thickness and the value calculated using the widely used industry formula.
For materials with high solids content, the discrepancy was approximately 6–7%, which could initially be perceived as an acceptable technological deviation. However, further analysis showed that the cause was systemic and inherent to the calculation method itself.
It was established that the conventional calculation model applies solids by weight without accounting for the volume of evaporating solvent. Once a volumetric calculation was adopted, the results aligned with the dry film thickness measured in practice.
The issue has practical significance for the design and acceptance of mastic roofing and waterproofing systems. SP 17.13330.2017 “Roofs” and standard project documentation specify the required thickness of the waterproofing layer — not the mass of material applied. When an incorrect calculation model is used, a contractor may fail to achieve the specified coating thickness even when the stated application rate is fully observed.
Liquid waterproofing mastics are sold almost universally by mass — in containers of 20–25 kg, with application rates quoted in kg/m². Yet what determines the performance of the coating is not the mass of applied material but the dry film thickness achieved after solvent evaporation.
The problem is that dry film thickness is commonly estimated using the following simplified formula:
At first glance this appears logical. However, for materials with low solids content it can introduce a significant error.
Dry content is defined as a mass fraction, whereas dry film thickness is a function of volume. When the solvent evaporates, the volumetric loss is disproportionately large relative to the mass loss, because the solvent is significantly less dense than the non-volatile binder matrix.
It is notable that the applicable standards — GOST 30693-2000 (General Technical Requirements for Roofing and Waterproofing Mastics) and GOST 26589-94 (Test Methods) — do not prescribe any method for calculating dry film thickness from material parameters. Regulatory and project practice in mastic roofing and waterproofing operates in terms of the thickness of the formed coating, yet neither standard describes how to convert the parameters of a liquid mastic into actual dry film thickness. The absence of a normative calculation method has led to the widespread use of a simplified model that can introduce a significant error for materials with low solids content.
Correct Formula
The physically correct approach requires subtracting the volume of evaporated solvent from the total wet volume of the applied material:
In expanded form:
where:
- m — total mass of the container,
- ρmix — density of the liquid mastic,
- msolv — mass of the volatile fraction,
- ρsolv — density of the solvent.
Worked Example
Material: Representative solvent-based waterproofing mastic
- Container: 25 kg
- Density of liquid mastic: 1.55 kg/l
- Dry content: 62%
- Volatile fraction: 38%
- Solvent density: 0.86 kg/l
Simplified conventional calculation:
25 × 0.62 / 1.55 = 10.0 l
This yields approximately 10.0 m² at a dry film thickness of 1 mm.
Correct volumetric calculation:
- Volume of liquid mastic: 25 / 1.55 = 16.13 l
- Mass of solvent: 25 × 0.38 = 9.5 kg
- Volume of solvent: 9.5 / 0.86 = 11.05 l
- Actual dry volume: 16.13 − 11.05 = 5.08 l
Summary
| Calculation method | Coverage at 1 mm dry film |
|---|---|
| Simplified calculation | 10.0 m² |
| Correct volumetric calculation | 5.1 m² |
Coverage Comparison Across Selected Products
Coverage from a 25 kg container at given dry film thickness (m²):
| Dry film thickness |
Hyperdesmo® | Liquid Roofing | Solvent-based mastic |
Elastomeric 110 | Elastomeric 911 | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| correct | simp. +7.5% | correct | simp. +28.5% | correct | simp. +96.8% | correct | simp. +32.3% | correct | simp. +22.1% | |
| 1 mm | 15.0 | 16.1 | 10.4 | 13.3 | 5.1 | 10.0 | 7.6 | 10.0 | 9.5 | 11.6 |
| 2 mm | 7.5 | 8.0 | 5.2 | 6.7 | 2.5 | 5.0 | 3.8 | 5.0 | 4.8 | 5.8 |
| 3 mm | 5.0 | 5.4 | 3.5 | 4.4 | 1.7 | 3.3 | 2.5 | 3.3 | 3.2 | 3.9 |
| 4 mm | 3.7 | 4.0 | 2.6 | 3.3 | 1.3 | 2.5 | 1.9 | 2.5 | 2.4 | 2.9 |
Note. “simp.” — simplified calculation using solids by weight. Source parameters (density, solids content, application rate) are taken from published technical data sheets of the respective manufacturers. Coverage and dry film thickness calculations were performed by the authors using two models: the simplified industry model and the volumetric model accounting for solvent density. A uniform solvent density model was applied across all materials for comparability.
Conclusion
If a liquid mastic contains a volatile solvent, dry film thickness cannot be correctly derived from solids by weight alone. The volume of evaporated solvent must be accounted for explicitly.
For the purposes of design, acceptance, and performance assessment of a coating, what matters is the actual dry film thickness achieved at a given application rate.